Filter circuit



`june 22, 1937. 1 MCKESSON n2,084,740

l FILTER CIRCUIT Filed Jan. 20, 1956 2 Sheets-Sheet l Lamb/ L /0,000 raw INVENTOR. LEWIS J. MCKESSON BY g/ufH/k/ ATTORNEY.

June'22, 1937. MCKESSON 2,084,740

FILTER CIRCUIT Filed Jan. 20,1956 Y 2 Sheets-Sheet 2 INVENTOR. LEWIS J. MCKESSON ATTORNEY.

Patented June 22, 1937 UNITED STATES PATENT OFFICE FILTER CIRCUIT of Delaware Application January 20, 1936, Serial No. 59,858

10 Claims.

This invention relates to electrical systems and especially concerns auxiliary equipment associated therewith for preventing undesired currents from affecting said systems and associated apparatus.

In transmission line systems where alternating currents are employed, particularly in high frequency radio transmitting systems, it is desirable to suppress certain currents originating in the systems themselves, such as harmonics of the frequency desired to be radiated orv absorbed, since these currents not only contribute to diminish the operating efiiciency but also radiate undesired waves. The presence of second and third harmonics of the fundamental frequency have been found to be particularly undesirable, especially in the short wave transmitter field where the problem of reducing radiation on harmonic frequencies has become increasingly important.

It is known that filter circuits, usually in the form of low pass filter arrangements, may be used to suppress separately each of the undesired harmonic frequencies, but such arrangements, in

a circuit where the second and third and higher harmonics are required to be suppressed, result in the use of a large number of inductances and capacitances. This is a decided disadvantage in high frequency transmission line systems.

The above mentioned difficulty is overcome in the present invention which provides a circuit utilizing the same inductances and capacitances to resonate at specified frequencies. In this manner the number of reactances used is reduced as compared with other circuits having the same electrical characteristics.

The present invention is particularly effective in high power radio transmitters which for the most part employ two ormore vacuum tubes in push-pull relationship, the output of which is coupled to a transmission line which is balanced to ground. In such systems, the second as well as other even harmonic voltages is present on the transmission line as a push-push voltage, that is, the voltage is zero across the line and maximum between each line and ground. The third, as well as other odd harmonic voltages on the line, however, appears as a push-pull voltage where the voltage is 180 maximum across the line.

According to the present invention, a circuit arrangement is proposed which provides, theoretically, infinite attenuation for second and third harmonic voltages, and is also highly effective in suppressing higher harmonic voltages.

The following is a more detailed description of (Cl. Z50- 17) the invention in conjunction with the drawings wherein: i

Fig. 1 illustrates, schematically and by way of example only, the harmonic filter circuit of the invention as applied to a high frequency radio 5 transmission line circuit;

Fig. 2 is a graph illustrating the effectiveness of the lter of the invention on the second and third harmonic frequencies; and

Figs. 3 and 4 show modifications of the harmonic filter circuit of the invention.

Although the invention is hereinafter described in connection with a transmission line circuit, it is to be distinctly understood that it is not limited thereto since the invention may also be used in other arrangements, such as in the anode circuits of push-pull amplifiers and oscillators, etc. Similarly, the invention is not limited to low pass filter circuits but can be used on high pass, band pass or band elimination filters. 2O

Referring to Fig. l, there is indicated diagrammatically in box I, a conventional type of pushpull power amplifier stage of a radio transmitter with which there are associated a pair of vacuum tubes and an output or tank circuit 2. Since the transmitting circuit forms no part of the present invention per se, and is of a type well-known in the art, no detailed description thereof will be given. Tank circuit 2 is inductively coupled to coil 3, which is associated with an antenna system 4 through transmission line 5 and filter circuit B. Connected to the center of coil 3 is a high resistance R2 which also connects with the anode supply for the transmitter tubes, as shown. Serially connected in the transmission line circuit 5 are a pair of blocking condensers C3, C4 of negligible impedance to the high frequency currents which prevent direct current from the positive terminal of the anode source of supply from entering the filter and antenna circuits. Resistor R2 functions to maintain coupling coil 3 at the transmitter direct current anode potential to prevent arcing from the tank coil 2 to the coupling coil 3, a condition which would otherwise occur due to leakage across the transmission line blocking condensers C3, C4. Inductance coils L1 and L2, generally known as drain coils, are of high impedance to the high frequency voltage and serve to maintain the antenna and lter circuits at ground potential for direct currents. 50

The filter circuit of the invention for suppressing second, third and higher harmonic voltages, and for enabling the desired degree of coupling between antenna and transmitter is shown between the two vertical dot and dash lines and labeled 6. Reactance coils La, Li, L5 and Le are adjustable and give the desired degree of coupling between line 5 and coil 3. Since the second harmonic voltage, indicated as 2j in Fig. 2, is present as a push-push voltage on the transmission line, and the third harmonic voltage, indicated as 3f in Fig. 2, is of push-pull nature on the line, it is required therefore, in order to attenuate the second harmonic, to provide a low impedance path between each side of the line and ground, and to attenuate the third harmonic it is required to provide a low impedance path across the line. Adjustable capacitances C5 and C6 and adjustable reactance coils L7, Ls and L9 are cmployed to achieve this result. Coils L7 and Ls may, for convenience, be combined to form a single coil whose mid-point connects with L9. Also, if desired, and in some instances it i s preferred, a capacitance C7 may replace inductance coil L9, as shown in Fig. 3. The choice between an inductance L9 or a capacitance C7 will depend on the constants of the other impedances in the circuit.

The invention is not limited to the arrangement of inductances and capacitances shown in Figs. l and 3, since these may be changed to the arrangement illustrated in Fig. 4. Although electrically the filter of Fig. 4 is the same as those of Figs. 1 and 3, the system of Fig. 4 may, in some instances, have practical advantages over the other arrangements.

In setting up the filter circuit, the transmitter l is first properly loaded in known manner by matching the impedance thereof with respect to the line 5. This is done by means of coils Ls, L4, L5 and La, and a mid-shunt capacitance C8 used to replace C5, C6, L7, La and L9 temporarily during adjustments. The shunt tuning circuit consisting of C5, Ce, L7, Ls and L9 is connected across the line replacing the temporary capacitance;

Filter 6 may be tuned to suppress the harmonics in the following manner:

(l) A selective receiver is first set up which is equipped with an output meter for indicating Volume, and the receiver tuned to one of the harmonic frequencies in the transmitter system, let us say the third harmonic. In setting up the receiver, care should be taken that it is coupled to the transmission line 5 or antenna 4, and not to the coils of the lter circuit 6. If desired, a Wavemeter may be used instead of the receiver, but the wavemeter is generally not as satisfactory as the receiver.

(2) The series circuit comprising C5, L7, La, Ce should now be resonated to the third harmonic. Resonance of this circuit will be indicated by a reduction in the receiver output, or an increase. in the wavemeter reading if coupled to L7, La. The tuning of this circuit, it will be found, is extremely sharp.

(3) The second harmonic circuits are now resonated by varying coil L9 whose adjustment will have no effect on the third harmonic trap tuning. If desired, the inductance of coils L7 and La may be reduced by employing a short-circuiting strap indicated by dotted lines S. This strap is symmetrically connected to the coils L7 and Ls with respect to the junction point to which coil L9 connects, in order that the second harmonic tuning be not affected.

If after tuning the second and third harmonic circuits, as above indicated, the loading of the transmission line and antenna on the coupling 3 and tank circuit 2 has been altered, this can be corrected by readjusting the loading coils La,

L4, L5 and Le. In adjusting the harmonic trap circuits, care should be exercised so that the tank circuit 2 remains exactly in tune, since slight detuning will cause a change in the amplitude of the harmonics.

The tuning of the filter circuit t will be correct when (a) the load on the power amplier transmitter circuit l is resistive only, and (b) the impedance of the output circuit is such that the power amplier is loaded to the desired value. Condition (a) will exist when the tuning of tank circuit 2 is very nearly the same with the transmission line 5 connected or disconnected from coil 3. When both of these conditions (a) and (b) are satised, the power amplifier efficiency will be a maximum for a given amount of loading.

One advantage of the filter circuit of the inn vention is that it provides a greater attenuation for the second harmonic voltage than certain circuits heretofore employed, inasmuch as the midshunt section C5, C5, L7, La, L9 is practically equivalent to two circuits in parallel as compared to a single circuit. Another advantage is that the iilter provides a Very large attenuation for the third harmonic voltages, and more attenuation on higher harmonics than obtained in previously used circuits.

In one embodiment constructed in accordance with the invention, the amplitude of the third harmonic was reduced from 37 niicrovolts to less than .5 microvolt, corresponding to a 45 decibel reduction. Equivalent results were obtained on second harmonic reduction.

Fig. l sets forth, by way of example only, values for the constants which may be used in a harn monic filter circuit associated with a short wave transmitter system adapted to function cn 4535 kc. The filter provides maximum attenuation to the second and third harmonic voltages and, in addition, acts as an impedance matching circuit between the transmitter I and antenna transmission line 5. In practice, it may be found that the calculated and actual values dc not agree very closely in a filter of this type when mounted in or near shielding becausev of the diiiculty of estimating or calculating stray capacities and mutual impedances, especially at high frequencies. Also, because of line reflections and other factors, the input and output impedances are known only approximately so the actual circuity values are best determined by trial and erro-i" after the approximate values are known.

What is claimed is:

1. The combination with a system having a two wire path adapted to be traversed with alternating currents, of a filter circuit for attenuating second and third harmonic voltages from said path, said filter comprising a series circuit of capacitance, inductance and capacitance, in the order named, across said wires of said path, and

another inductance connected effectively be` tween the electrical center of said first inductance and ground.

2. An arrangement in accordance with claim 1, characterized in this that means are provided for adjusting the values of each of said inductance and capacitance elements, said series circuit being tuned to resonance for suppressing the third harmonic voltages and said last inductance element being adjusted to aid in attenuating said second harmonic voltage.

3. The combination with an electrical system having a two wire line including an input circuit and an output circuit connected to opposite ends of said line, of a filter circuit intermediate said input and output circuits, said filter comprising an impedance matching Vcircuit having an inductance element connected in series in each wire of said line, and a harmonic voltage attenuator connected in shunt across said line, said attenuator comprising a series circuit of capacitance, inductance and capacitance, in the order named, tuned to the third harmonic voltage, and an inductance connected between the ends of said last inductance and ground and tuned to attenuate said second harmonic voltage.

4. In combination, an, antenna, an exciter circuit comprising a pair of push-pull electron discharge devices, a transmission line connected between said exciter circuit and said antenna, and a harmonic iilter circuit coupled to said transmission line, said lter comprising a series circuit of capacitance, inductance and capacitance, in the order named, connected in shunt to said line, said series circuit being tuned to resonate at the third harmonic voltage of the fundamental of said exciter circuit, and a variable connection to groundirom the electrical center of said inductance.

5. The combination with a short wave radio transmitter having a power ampliiier comprising a pair of push-pull electron discharge 'devices, of a load and a two wire transmission line connected between said load and ampliiier, a pair of adjustable coils in series in each side of said line for matching the impedance of said transmitter, and a series circuit of capacitance, inductance and capacitance, in the order named, connected between the junction point of one of said pairs of adjustable coils and the junction point of the other pair of adjustable coils, said last series circuit being tuned to resonance at the third harmonic voltage appearing in said line, and an adjustable reactance connected be'- tween ground and the inductance of said last series circuit for attenuating the second harmonic voltage appearing in said line.

6. A system in accordance with claim 5, characterized in this that said load is an antenna, said inductance in said series tuned circuit comprises a pair of coils, and said last adjustable reactance connects between ground and the junction point of said last pair of coils.

7. rIhe combination with a system having a two wire path adapted to be traversed with alternating currents, of a ilter circuit for attenuating second and third harmonic voltages from said path, said lter comprising a series circuit of capacitance, inductance and capacitance, in the order named, across said wires of said path, and another capacitance connected effectively between the electrical center of said inductance and ground.

8. The combination with an electrical system having a two wire line including an input circuit and an output circuit connected to opposite ends of said line, of a filter circuit intermediate said input and output circuits, said filter comprising an impedance matching circuit having an inductance element connected in series in each wire of said line, and a harmonic voltage attenuator connected in shunt across said line, said attenuator comprising a series circuit of capacitance, inductance and capacitance, in the order named, tuned to the third harmonic voltage, and a capacitance connected to a point between the ends of said last inductance and ground and tuned to attenuate said second harmonic voltage.

9. The combination with a system having a two wire path adapted to be traversed with alternating currents, of a iilter circuit for attenuating second and third harmonic voltages from said path, said iilter comprising a series circuit of inductance and capacitance across said wires, and a reactance connected between ground and the electrical center of said series circuit, whereby each half of said series circuit is symmetrically positioned between each wire of said path and ground.

10|. In combination, an antenna, an exciter circuit comprising a pair of push-pull electron discharge devices, a transmission line connected between said exciter circuit and said antenna, and a harmonic filter circuit coupled to said transmission line, said filter comprising a series circuit of capacitance, inductance and capacitance, in the order named, connected in shunt to said line, said series circuit being tuned to resonate at the third harmonic voltage of the fundamental of said exciter circuit, and a connection to ground from the electrical center of said inductance, said connection including a variable reactance.

LEWIS J. MCKESSON. 

